This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. SPECIFIC AIMS Primary Objective To evaluate the virologic activity over 14 days of three dose levels of SCH 417690 in HIV-infected, treatment-experienced subjects who are failing their current ritonavir-containing antiretroviral regimen. Secondary Objectives [unreadable] To assess the longer-term safety and tolerability of SCH 417690. [unreadable] To evaluate the virologic activity at 24 and 48 weeks of three doses of SCH 417690 together with a background ritonavir-containing antiretroviral regimen optimized on the basis of baseline genotypic and phenotypic testing. [unreadable] To assess baseline coreceptor tropism phenotype in the screening patient population, changes in coreceptor phenotype (i.e., from detection of CCR5-only virus to detection of CCR5/CXCR4 or to CXCR4-only virus) in subjects taking SCH 417690, and whether the presence of X4 viral strains as a minority quasi-species influences the virologic response to SCH 417690. [unreadable] To explore change in susceptibility to SCH 417690 and the background antiretroviral regimen, and changes in viral fitness in subjects receiving SCH 417690 who experience virologic failure. [unreadable] To explore the effect of past and/or current enfuvirtide use on virologic response in subjects receiving SCH 417690. [unreadable] To evaluate the immunologic activity at 24 and 48 weeks of SCH 417690 together with an optimized background ritonavir-containing antiretroviral regimen. [unreadable] To explore the effects of SCH 417690 on immune system responses and the incidence of clinical infectious diseases. [unreadable] To explore the relationship of SCH 417690 concentrations at Day 14 and Week 8 to virologic response at Week 4 and at Week 24 (or earlier study drug termination), respectively. [unreadable] To explore the relationship of adherence with SCH 417690 and background antiretrovirals to virologic response.